Sunday, 20 January 2013

Homemade Axial Flux Generator - Part 1 - The Mechanics....

Late last year, I'd thought I'd have a go at building an experimental generator which could be used with a wind turbine, either a new-build Savonius vertical-axis type or else utilising my spare set of home-made blades for our existing horizontal-axis machine.     

I'm starting small, at only around 150 mm (6") overall rotor diameter but I've a couple of worn front brake discs lying around in the workshop from my old MGF (long gone now....) which could possibly be used for a larger version in the future.

After first looking around at what others have done on YouTube etc, I came up with a slightly different design to most of the others and then set to work.    At this stage it's intended to be a 3-phase alternator and so with 12 magnet poles we'll need 9 coils cutting the flux during rotation.  This will give the required 120 degrees of phase separation during operation - there's several animations available on the web which show moving graphs of the voltages induced in a 3-phase alternator.

The generator coils will be static, i.e. will form the 'stator', and the magnets are fixed to steel plates and rotate, i.e. the 'rotor'.  The assembled rotor comprises two parts, the outer and inner.  The rotor shaft rotates in ball-bearings in the housing, to which the stator will be fixed in position.

I bought a couple of profiled steel plates from eBay, for the rotor discs.   I already had several pieces of aluminium round and square bar in the workshop from which to machine the bearing housings and shafts etc.

bearing housing, temporary central bolt,
rear bearing, rear stub shaft and bearing spacer

There are two bearings fitted in the front of the housing, so the generator shaft can rotate independently, and a third at the rear which will support the direct connection of a turbine blade set via a stub shaft.   The entire blade hub / generator assembly could then be retained by a single M10 central bolt - I've just used a length of studding for the moment, turned down to 8 mm diameter at the rear end so I can fit into the cordless drill chuck for testing.

The cylindrical bearing spacer fits over the shafts and provides contact between the inner races of the front and rear bearing sets.  The spacer is slightly longer than the distance between the bearing bore faces in the housing.  This allows the central bolt to be fully tightened without overloading the bearings and causing them to lock-up, as would be the tendency if the load path was taken across the ball elements by tightening over both the inner and outer raceways.  An alternative would be to use a length of studding as a central shaft and adjust the axial load on the bearings during assembly so they're free to rotate without excessive end-float, and then fit locknuts to retain the shaft in the set position.

The four tapped holes shown on the face of the housing are to fix the stator assembly.  I drilled sets of tapped holes on all the other housing faces to give a variety of mounting options for the assembled generator without having to take it apart and re-machine it in future.

The lower profiled block currently shown attached under the bearing housing can be bored out at its base to fit directly onto a turbine mast pole if it's to be used for a horizontal-axis machine - for the moment, it simply allows me to hold the generator in the vice for assembly and testing.

Again from eBay, I bought 50 neodymium disc magnets at 20 mm diameter and 5 mm thickness.   At £39, these were from N35 material, the lowest grade available but 50 x N35s were around the same price as 25 x N40s and I needed 48 of them in the design.

On the CNC machine, I first machined pockets in the steel plates for the first column of magnets, and also drilled the mounting and fixing holes.   Once this machining was completed, I match-marked and assembled the rotor plates together with the shaft and trued-up the outside diameters on the lathe, so they were both concentric with the shaft - this should help ensure the rotor assembly is already close to being balanced, although I may still need to add a few balancing weights during testing to eliminate any vibration.

Twelve magnets were then fitted into the pockets of each rotor plate with epoxy, and then the next column of twelve magnets was placed on top.

outer rotor plate, with the first sets of magnets fixed
and showing shaft mounting bore & holes
and the inner rotor plate at the same stage....

One word of caution, is that neodymium magnets are very strong and they must be handled with respect at all times.   If you allow steel tools etc to become too close to the magnet surfaces, then they'll come together very sharply and there's a risk of damaging the magnets.

The magnet poles are arranged alternately North-South around the discs, and the fixing holes in each rotor plate are drilled and match-marked to ensure that the North pole of magnets on the outer rotor plate is opposed on the inner plate by a South pole, and vice versa.  This arrangement retains the flux within the rotor plates and across the gap between - it's very effective, touching the back faces of the plates with a screwdriver shows that there's very little magnetic force there at all, but the front faces are very strongly magnetic.

Then, to retain the outer set of magnets, I made a mould around the centre of each disc, mixed up some polyurethane casting resin and then poured it in to encapsulate the magnets.

second column of magnets and shaft fitted  (the outer screws
are temporary for moulding purposes only)

mould for the outer rotor.....

after pouring, solidification and demoulding....
the magnets are just below the surface of the resin

The two-part polyurethane casting resin was bought on eBay, quite expensive at £21 delivered for just one litre, but it's a fast-cast system with a potlife of just a few minutes and ready to demould in less than half an hour.  There's more than enough for the two rotors and also to fully encapsulate the stator coils.

trial assembly of the mechanical components

The above photo is of a trial assembly of the generator - at this stage, all the mechanical components are basically complete, except for a spray coat of Hammerite or similar on the rotors to guard against corrosion of the steel parts.  The studs and nuts I've used are all from stainless steel.  The magnet surfaces are already covered with polyurethane - it's important that they're not exposed to the elements, since even though they're supposed to be nickel plated during manufacture, my previous experience with similar magnets is that they can very soon show signs of corrosion.

You can see I've used lengths of studding to separate the two halves of the rotor.  This allows the gap between, and hence the clearance between the magnets and the stator coils, to be adjusted to a minimum on final assembly.  The closer the magnets are to the coils, the better the generator performance.

clearance between the inner rotor plate and bearing housing...

From the above photo, you can see the clearance between the inner rotor and the square bearing housing.  This allows the rotor to turn freely without fouling the housing and also provides a cable route to pass the wires from the stator through the inner rotor bore.

So, now that it's mechanically complete, I'll move onto the tricky part of winding the coils etc.   I've already bought the winding wire and 3-phase bridge rectifier, and started on making coil formers etc - I'll show the construction of the stator and initial testing of the finished generator in a future post....

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